THS4500 Datasheet, PDF (28/37 Pages) Texas Instruments – WIDEBAND, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIERS

THS4500
THS4501
SLOS350D − APRIL 2002 − REVISED JANUARY 2004
As can be seen in the equation, when a higher impedance
is used, the same level of intermodulation distortion
performance results in a lower intercept point. Therefore,
it is important to comprehend the impedance seen by the
output of the fully differential amplifier when selecting a
minimum intercept point. The graphic below shows the
relationship between the strict definition of an intercept
point with a normalized, or equivalent, intercept point for
the THS4502.
THIRD-ORDER OUTPUT INTERCEPT POINT
vs
FREQUENCY
60
55
Normalized to 200 Ω
Normalized to 50 Ω
50
45
40
35
OIP3 RL= 800 Ω
30
Gain = 1
25 _{Rf = 392 Ω}
20
VS = ± 5 V
Tone Spacing = 200 kHz
15
0 10 20 30 40 50 60 70 80 90 100
f − Frequency − MHz
Figure 109
Comparing specifications between different device types
becomes easier when a common impedance level is
assumed. For this reason, the intercept points on the
THS4500 family of devices are reported normalized to a
50-Ω load impedance.
AN ANALYSIS OF NOISE IN FULLY
DIFFERENTIAL AMPLIFIERS
Noise analysis in fully differential amplifiers is analogous
to noise analysis in single-ended amplifiers. The same
concepts apply. Below, a generic circuit diagram
consisting of a voltage source, a termination resistor, two
gain setting resistors, two feedback resistors, and a fully
differential amplifier is shown, including all the relevant
noise sources. From this circuit, the noise factor (F) and
noise figure (NF) are calculated. The figures indicate the
appropriate scaling factor for each of the noise sources in
two different cases. The first case includes the termination
resistor, and the second, simplified case assumes that the
voltage source is properly terminated by the gain-setting
resistors. With these scaling factors, the amplifier’s input
noise power (N_A) can be calculated by summing each
individual noise source with its scaling factor. The noise
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delivered to the amplifier by the source (N_I) and input noise
power are used to calculate the noise factor and noise
figure as shown in equations 23 through 27.
Ni NA
eg _Rg
Rf
ef
Si
en
Ni
No
Rs
+
Rt
ini
fully-diff
amp
So
No
es
−
et
iii
eg
Rg
Rf
ef
Figure 110. Noise Sources in a Fully
Differential Amplifier Circuit
NA: Fully Differential Amplifier
Noise
Source Scale Factor
(eni)2
2
ȡ ȣ _Rg
ȧȢ ȧȤ _Rf
)
R_g
)
R_g
R_sR_t
2ǒR_s)R_tǓ
(12)
(ini)2
Rg2
(iii)2
Rg2
(13)
(14)
ȡ ȣ ^2RsRG
2
4kTRt
R_s)2R_g
ȧȢ ȧȤ _Rt
)
2R_sR_g
R_s)2R_g
(15)
4kTRf
ǒ Ǔ 2
R_g ²
R_f
(16)
È¡
R_g
È£²
4kTRg
ȧȢ ȧȤ ²
R_g
)
R_sR_t
2ǒR_s)R_tǓ
(17)
Figure 111. Scaling Factors for Individual Noise
Sources Assuming a Finite Value Termination
Resistor
28